diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 11f784c..b4e9b04 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -6,6 +6,7 @@ import { BrowserRouter,
 	 Routes,
 	 useLocation } from 'react-router-dom'
 
+import DevModeWatermark from '@/components/DevModeWatermark'
 import RouteBlockerOverlay from '@/components/RouteBlockerOverlay'
 import TopNav from '@/components/TopNav'
 import DialogueProvider from '@/components/dialogues/DialogueProvider'
@@ -145,6 +146,7 @@ const App: FC = () => {
   return (
       <>
 	<RouteBlockerOverlay/>
+	{import.meta.env.DEV && <DevModeWatermark/>}
 
 	<BrowserRouter>
 	  <DialogueProvider>
@@ -152,7 +154,7 @@ const App: FC = () => {
 	      <motion.div
 		  layout="position"
 		  transition={{ layout: { duration: .2, ease: 'easeOut' } }}
-		  className="flex flex-col h-dvh w-full overflow-y-hidden">
+		  className="relative flex flex-col h-dvh w-full overflow-y-hidden">
 		<TopNav user={user}/>
 		<RouteTransitionWrapper user={user} setUser={setUser}/>
 	      </motion.div>
diff --git a/frontend/src/components/DevModeWatermark.tsx b/frontend/src/components/DevModeWatermark.tsx
new file mode 100644
index 0000000..5de8c0b
--- /dev/null
+++ b/frontend/src/components/DevModeWatermark.tsx
@@ -0,0 +1,46 @@
+import nikumaru from '@/assets/fonts/nikumaru.otf'
+
+import type { FC } from 'react'
+
+
+const ROW_COUNT = 8
+const COLUMN_COUNT = 12
+
+
+const DevModeWatermark: FC = () => {
+  return (
+      <div
+	  aria-hidden="true"
+	  className="pointer-events-none select-none fixed inset-0 overflow-hidden z-0">
+	<style>{`
+	  @font-face {
+	    font-family: 'Nikumaru';
+	    src: url(${nikumaru}) format('opentype');
+	  }
+	`}</style>
+	<div className="absolute -inset-32 flex flex-col justify-center gap-12 py-48">
+	  {Array.from ({ length: ROW_COUNT }, (_, rowIndex) => (
+	      <div
+		  key={rowIndex}
+		  className={
+		      'flex min-h-32 items-center gap-12 '
+		      + (rowIndex % 2 === 0 ? 'translate-x-0' : 'translate-x-32')
+		  }>
+		{Array.from ({ length: COLUMN_COUNT }, (_, columnIndex) => (
+		    <span
+			key={columnIndex}
+			className={
+			    'whitespace-nowrap text-3xl font-bold '
+			    + 'tracking-[0.3em] text-neutral-950/5 '
+			    + 'dark:text-white/10'
+			}
+			style={{ fontFamily: 'Nikumaru' }}>
+		      開発モード
+		    </span>))}
+	      </div>))}
+	</div>
+      </div>)
+}
+
+
+export default DevModeWatermark
diff --git a/frontend/src/components/DraggableDroppableTagRow.tsx b/frontend/src/components/DraggableDroppableTagRow.tsx
index e6c49ff..97e8248 100644
--- a/frontend/src/components/DraggableDroppableTagRow.tsx
+++ b/frontend/src/components/DraggableDroppableTagRow.tsx
@@ -87,10 +87,13 @@ const DraggableDroppableTagRow: FC<Props> = ({ tag, nestLevel, pathKey, parentTa
 	    setDropRef (node)
 	  }}
 	  style={style}
-	  className={cn ('rounded select-none', over && 'ring-2 ring-offset-2')}
+	  className={cn (
+	      'min-w-0 max-w-full overflow-hidden rounded select-none',
+	      over && 'ring-2 ring-offset-2')}
 	  {...attributes}
 	  {...listeners}>
 	<motion.div
+	    className="flex min-w-0 max-w-full items-baseline overflow-hidden"
 	    transition={{ layout: { duration: .2, ease: 'easeOut' } }}
 	    layoutId={`tag-${ sp ? 'sp-' : '' }${ tag.id }`}>
 	  <TagLink tag={tag} nestLevel={nestLevel}/>
@@ -98,4 +101,4 @@ const DraggableDroppableTagRow: FC<Props> = ({ tag, nestLevel, pathKey, parentTa
       </div>)
 }
 
-export default DraggableDroppableTagRow
\ No newline at end of file
+export default DraggableDroppableTagRow
diff --git a/frontend/src/components/MaterialSidebar.tsx b/frontend/src/components/MaterialSidebar.tsx
index 399d9d8..8428c09 100644
--- a/frontend/src/components/MaterialSidebar.tsx
+++ b/frontend/src/components/MaterialSidebar.tsx
@@ -318,12 +318,20 @@ const MobileMaterialTreeNode: FC<{ depth?: number
 	    <TagLink
 		tag={sidebarTagToTag (tag)}
 		title={tag.name}
+		truncateOnMobile
 		withCount={false}
 		withWiki={false}
 		to={materialPath (tag.id, materialFilter)}
 		className="block overflow-hidden text-ellipsis whitespace-nowrap
 			   [max-inline-size:var(--tag-link-inline-size)]
-			   [max-height:var(--tag-link-inline-size)]"/>
+			   [max-height:var(--tag-link-inline-size)]
+			   [&_.tag-marquee]:block
+			   [&_.tag-marquee]:h-full
+			   [&_.tag-marquee]:overflow-hidden
+			   [&_.tag-marquee__static]:h-full
+			   [&_.tag-marquee__static]:overflow-hidden
+			   [&_.tag-marquee__static]:text-ellipsis
+			   [&_.tag-marquee__static]:[text-overflow:ellipsis]"/>
 	  </div>
 	  {tag.hasChildren && (
 	      <button
diff --git a/frontend/src/components/ResponsiveMarqueeText.tsx b/frontend/src/components/ResponsiveMarqueeText.tsx
new file mode 100644
index 0000000..c859055
--- /dev/null
+++ b/frontend/src/components/ResponsiveMarqueeText.tsx
@@ -0,0 +1,250 @@
+import { cn } from '@/lib/utils'
+import { useEffect, useRef, useState } from 'react'
+
+import type { FC } from 'react'
+
+type Props = {
+    text:      string
+    className?: string
+    truncateOnMobile?: boolean
+    title?:    string }
+
+const DESKTOP_MARQUEE_MEDIA =
+    '(min-width: 768px) and (hover: hover) and (pointer: fine) and (prefers-reduced-motion: no-preference)'
+const MARQUEE_START_DELAY_MS = 1600
+const MARQUEE_END_HOLD_MS = 2400
+const MARQUEE_SCROLL_PX_PER_SECOND = 43
+const MIN_MARQUEE_OVERFLOW_PX = 1
+
+
+const ResponsiveMarqueeText: FC<Props> = (
+    { text, className, truncateOnMobile = false, title },
+) => {
+  const outerRef = useRef<HTMLSpanElement | null> (null)
+  const staticRef = useRef<HTMLSpanElement | null> (null)
+  const animatedRef = useRef<HTMLSpanElement | null> (null)
+  const animationRef = useRef<Animation | null> (null)
+  const timeoutRef = useRef<number | null> (null)
+  const rafRef = useRef<number | null> (null)
+  const [overflowPx, setOverflowPx] = useState (0)
+  const [desktopMarqueeEnabled, setDesktopMarqueeEnabled] = useState (false)
+  const [active, setActive] = useState (false)
+  const [marqueeVisible, setMarqueeVisible] = useState (false)
+
+  useEffect (() => {
+    const outer = outerRef.current
+    const inner = staticRef.current
+
+    if (!(outer) || !(inner))
+      return
+
+    const measure = () => {
+      const nextOverflow = Math.max (0, Math.ceil (inner.scrollWidth - outer.clientWidth))
+      setOverflowPx (prev => Math.abs (prev - nextOverflow) <= 1 ? prev : nextOverflow)
+    }
+
+    measure ()
+
+    const resizeObserver =
+	typeof ResizeObserver === 'undefined'
+	? null
+	: new ResizeObserver (() => {
+	    measure ()
+	  })
+
+    resizeObserver?.observe (outer)
+    resizeObserver?.observe (inner)
+    addEventListener ('resize', measure)
+
+    return () => {
+      resizeObserver?.disconnect ()
+      removeEventListener ('resize', measure)
+    }
+  }, [text])
+
+  useEffect (() => {
+    if (typeof window === 'undefined' || typeof window.matchMedia !== 'function')
+      return
+
+    const media = window.matchMedia (DESKTOP_MARQUEE_MEDIA)
+    const update = () => {
+      setDesktopMarqueeEnabled (media.matches)
+    }
+
+    update ()
+    media.addEventListener ('change', update)
+
+    return () => {
+      media.removeEventListener ('change', update)
+    }
+  }, [])
+
+  useEffect (() => {
+    const clearScheduled = () => {
+      if (timeoutRef.current != null)
+        {
+          clearTimeout (timeoutRef.current)
+          timeoutRef.current = null
+        }
+      if (rafRef.current != null)
+        {
+          cancelAnimationFrame (rafRef.current)
+          rafRef.current = null
+        }
+      animationRef.current?.cancel ()
+      animationRef.current = null
+    }
+
+    const animated = animatedRef.current
+    const canMarquee = (
+        active
+        && desktopMarqueeEnabled
+        && overflowPx >= MIN_MARQUEE_OVERFLOW_PX
+        && animated != null)
+
+    const resetAnimated = () => {
+      const node = animatedRef.current
+
+      animationRef.current?.cancel ()
+      animationRef.current = null
+      node?.getAnimations?.().forEach (animation => {
+        animation.cancel ()
+      })
+
+      if (!(node))
+        return
+
+      node.style.transition = 'none'
+      node.style.transform = 'translateX(0)'
+    }
+
+    if (!canMarquee)
+      {
+        clearScheduled ()
+        resetAnimated ()
+        setMarqueeVisible (false)
+        return
+      }
+
+    let cancelled = false
+
+    const sleep = (ms: number) =>
+	new Promise<void> (resolve => {
+	  timeoutRef.current = window.setTimeout (() => {
+	    timeoutRef.current = null
+	    resolve ()
+	  }, ms)
+	})
+
+    const runLoop = async () => {
+      while (!cancelled)
+        {
+          resetAnimated ()
+          setMarqueeVisible (true)
+
+          await sleep (MARQUEE_START_DELAY_MS)
+          if (cancelled || !(animatedRef.current))
+            break
+
+          const moveDurationMs =
+	      overflowPx / MARQUEE_SCROLL_PX_PER_SECOND * 1000
+          const node = animatedRef.current
+
+          if (typeof node.animate === 'function')
+            {
+              const animation = node.animate (
+		  [
+		    { transform: 'translateX(0)' },
+		    { transform: `translateX(-${ overflowPx }px)` },
+		  ],
+		  { duration: moveDurationMs, easing: 'linear', fill: 'forwards' })
+              animationRef.current = animation
+              try
+              {
+                await animation.finished
+              }
+              catch
+              {
+                break
+              }
+
+              node.style.transform = `translateX(-${ overflowPx }px)`
+              animation.cancel ()
+              if (animationRef.current === animation)
+                animationRef.current = null
+            }
+          else
+            {
+              node.style.transition = `transform ${ moveDurationMs }ms linear`
+              await new Promise<void> (resolve => {
+                rafRef.current = requestAnimationFrame (() => {
+                  rafRef.current = null
+                  node.style.transform = `translateX(-${ overflowPx }px)`
+                  resolve ()
+                })
+              })
+              await sleep (moveDurationMs)
+              node.style.transition = 'none'
+              node.style.transform = `translateX(-${ overflowPx }px)`
+            }
+
+          if (cancelled)
+            break
+
+          await sleep (MARQUEE_END_HOLD_MS)
+          if (cancelled)
+            break
+
+          resetAnimated ()
+        }
+    }
+
+    void runLoop ()
+
+    return () => {
+      cancelled = true
+      clearScheduled ()
+      resetAnimated ()
+      setMarqueeVisible (false)
+    }
+  }, [active, desktopMarqueeEnabled, overflowPx, text])
+
+  return (
+      <span
+	  ref={outerRef}
+	  title={title ?? text}
+	  onMouseEnter={() => setActive (true)}
+	  onMouseLeave={() => setActive (false)}
+	  onFocus={() => setActive (true)}
+	  onBlur={() => setActive (false)}
+	  className={cn (
+	      'tag-marquee inline-block max-w-full min-w-0 align-bottom',
+	      (truncateOnMobile
+	       ? 'overflow-hidden text-ellipsis whitespace-nowrap'
+	       : 'whitespace-normal [overflow-wrap:anywhere]'),
+	      'md:overflow-hidden md:whitespace-nowrap',
+	      className)}>
+	<span
+	    ref={staticRef}
+	    className={cn (
+		'tag-marquee__static block max-w-full',
+		(truncateOnMobile
+		 ? 'overflow-hidden text-ellipsis whitespace-nowrap'
+		 : 'whitespace-normal [overflow-wrap:anywhere]'),
+		'md:overflow-hidden md:text-ellipsis md:whitespace-nowrap',
+		marqueeVisible && 'md:opacity-0')}>
+	  {text}
+	</span>
+	{overflowPx >= MIN_MARQUEE_OVERFLOW_PX && (
+	    <span
+		ref={animatedRef}
+		aria-hidden="true"
+		className={cn (
+		    'tag-marquee__animated hidden md:block',
+		    marqueeVisible ? 'md:opacity-100' : 'md:opacity-0')}>
+	      {text}
+	    </span>)}
+      </span>)
+}
+
+export default ResponsiveMarqueeText
diff --git a/frontend/src/components/TagLink.tsx b/frontend/src/components/TagLink.tsx
index a32dd4a..180892b 100644
--- a/frontend/src/components/TagLink.tsx
+++ b/frontend/src/components/TagLink.tsx
@@ -1,4 +1,5 @@
 import PrefetchLink from '@/components/PrefetchLink'
+import ResponsiveMarqueeText from '@/components/ResponsiveMarqueeText'
 import { LIGHT_COLOUR_SHADE, DARK_COLOUR_SHADE, TAG_COLOUR } from '@/consts'
 import { cn } from '@/lib/utils'
 
@@ -9,6 +10,7 @@ import type { Tag } from '@/types'
 type CommonProps = { 
     tag:        Tag
     nestLevel?: number
+    truncateOnMobile?: boolean
     withWiki?:  boolean
     withCount?: boolean }
 
@@ -30,9 +32,11 @@ type Props =
 const TagLink: FC<Props> = ({ tag,
 			      nestLevel = 0,
 			      linkFlg   = true,
+			      truncateOnMobile = false,
 			      withWiki  = true,
 			      withCount = true,
 			      className,
+			      title,
 			      ...props }) => {
   const spanClass = cn (
       `text-${ TAG_COLOUR[tag.category] }-${ LIGHT_COLOUR_SHADE }`,
@@ -41,11 +45,18 @@ const TagLink: FC<Props> = ({ tag,
       spanClass,
       `hover:text-${ TAG_COLOUR[tag.category] }-${ LIGHT_COLOUR_SHADE - 200 }`,
       `dark:hover:text-${ TAG_COLOUR[tag.category] }-${ DARK_COLOUR_SHADE - 200 }`)
+  const textClass = 'group min-w-0 max-w-full overflow-hidden align-bottom'
+  const rootClass =
+      'inline-flex min-w-0 max-w-full flex-nowrap items-stretch align-baseline gap-x-1 md:items-baseline'
+  const markerWrapClass = 'shrink-0 self-start md:self-auto'
+  const countClass = 'shrink-0 self-end md:self-auto'
+  const textTitle = title
+      ?? (tag.matchedAlias == null ? tag.name : `${ tag.matchedAlias } → ${ tag.name }`)
 
   return (
-      <>
+      <span className={rootClass}>
 	{(linkFlg && withWiki) && (
-	    <span className="mr-1">
+	    <span className={markerWrapClass}>
 	      {(tag.materialId != null || tag.hasWiki || tag.hasDeerjikists)
 	       ? (
 		  tag.materialId == null && !(tag.hasDeerjikists)
@@ -100,33 +111,48 @@ const TagLink: FC<Props> = ({ tag,
 	    </span>)}
 	{nestLevel > 0 && (
 	    <span
-		className="ml-1 mr-1"
+		className="ml-1 mr-1 shrink-0"
 		style={{ paddingLeft: `${ (nestLevel - 1) }rem` }}>
 	      ↳
 	    </span>)}
 	{tag.matchedAlias != null && (
 	    <>
-	      <span className={cn (spanClass, className)} {...props}>
-		{tag.matchedAlias}
+	      <span
+		  title={textTitle}
+		  className={cn (spanClass, textClass, className)}
+		  {...props}>
+		<ResponsiveMarqueeText
+		    text={tag.matchedAlias}
+		    title={textTitle}
+		    truncateOnMobile={truncateOnMobile}/>
 	      </span>
-	      <> → </>
+	      <span className="shrink-0"> → </span>
 	    </>)}
 	{linkFlg
 	 ? (
 	    <PrefetchLink
 		to={`/posts?${ (new URLSearchParams ({ tags: tag.name })).toString () }`}
-		className={cn (linkClass, className)}
+		title={textTitle}
+		className={cn (linkClass, textClass, className)}
 		{...props}>
-	      {tag.name}
+	      <ResponsiveMarqueeText
+		  text={tag.name}
+		  title={textTitle}
+		  truncateOnMobile={truncateOnMobile}/>
 	    </PrefetchLink>)
 	 : (
-	    <span className={cn (spanClass, className)}
+	    <span
+		  title={textTitle}
+		  className={cn (spanClass, textClass, className)}
 		  {...props}>
-	      {tag.name}
+	      <ResponsiveMarqueeText
+		  text={tag.name}
+		  title={textTitle}
+		  truncateOnMobile={truncateOnMobile}/>
 	    </span>)}
 	{withCount && (
-	    <span className="ml-1">{tag.postCount}</span>)}
-      </>)
+	    <span className={countClass}>{tag.postCount}</span>)}
+      </span>)
 }
 
 export default TagLink
diff --git a/frontend/src/components/TagSearchBox.tsx b/frontend/src/components/TagSearchBox.tsx
index 795daee..fd64cf3 100644
--- a/frontend/src/components/TagSearchBox.tsx
+++ b/frontend/src/components/TagSearchBox.tsx
@@ -20,12 +20,14 @@ const TagSearchBox: FC<Props> = ({ suggestions, activeIndex, onSelect }) => {
 		     rounded shadow">
 	{suggestions.map ((tag, i) => (
 	    <li key={tag.id}
-		className={cn ('px-3 py-2 cursor-pointer hover:bg-gray-300 dark:hover:bg-gray-700', 
+		className={cn ('min-w-0 overflow-hidden px-3 py-2 cursor-pointer hover:bg-gray-300 dark:hover:bg-gray-700',
 			       i === activeIndex && 'bg-gray-300 dark:bg-gray-700')}
 		onMouseDown={() => onSelect (tag)}>
-	      <TagLink tag={tag} linkFlg={false} withWiki={false}/>
+	      <div className="flex min-w-0 max-w-full items-baseline overflow-hidden">
+		<TagLink tag={tag} linkFlg={false} withWiki={false}/>
+	      </div>
 	    </li>))}
       </ul>)
 }
 
-export default TagSearchBox
\ No newline at end of file
+export default TagSearchBox
diff --git a/frontend/src/components/TagSidebar.tsx b/frontend/src/components/TagSidebar.tsx
index 9c08ccc..46fd6fe 100644
--- a/frontend/src/components/TagSidebar.tsx
+++ b/frontend/src/components/TagSidebar.tsx
@@ -64,8 +64,9 @@ const TagSidebar: FC<Props> = ({ posts, onClick }) => {
 	<ul>
 	  {CATEGORIES.flatMap (cat => cat in tags ? (
 	      tags[cat].map (tag => (
-		  <li key={tag.id} className="mb-1">
+		  <li key={tag.id} className="mb-1 min-w-0 max-w-full overflow-hidden">
 		    <motion.div
+			className="flex min-w-0 max-w-full items-baseline overflow-hidden"
 			transition={{ layout: { duration: .2, ease: 'easeOut' } }}
 			layoutId={`tag-${ tag.id }`}>
 		      <TagLink tag={tag} onClick={onClick}/>
@@ -128,4 +129,4 @@ const TagSidebar: FC<Props> = ({ posts, onClick }) => {
       </SidebarComponent>)
 }
 
-export default TagSidebar
\ No newline at end of file
+export default TagSidebar
diff --git a/frontend/src/index.css b/frontend/src/index.css
index f3709a4..e70403a 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -132,3 +132,20 @@ body
   0%, 100% { color: #f87171; }
   50% { color: #60a5fa; }
 }
+
+.tag-marquee
+{
+  position: relative;
+}
+
+.tag-marquee__animated
+{
+  position: absolute;
+  inset: 0 auto 0 0;
+  min-width: 100%;
+  width: max-content;
+  opacity: 0;
+  pointer-events: none;
+  white-space: nowrap;
+  will-change: transform;
+}